The present invention pertains to the control of the air supply to a burner such as a natural gas burner used in the production and/or processing of liquid and gaseous petroleum products. The amount of fuel, such as natural gas, consumed represents a reduction in the quantity available for sale and other uses. However, the typical burner with no variable air controller utilizes an excessive amount of air and excess oxygen which is inefficient and wasteful since unnecessary air is being heated.
The variable air controller described in our copending patent application, Ser. No. 06/347,393, is directed to controlling the total air flowing into the burner housing as a function of the pressure of the natural gas supplied to the burner. The structure of that application controls the amount of air to provide the desired 3% to 5% excess oxygen and achieved a significant savings of fuel gas. That invention was successful on units having small fire tubes, that is, four to eight inches, and particularly on units with a throttling temperature control. However, on larger units, particularly where the main burner is snapped on and off, irregular burning and flame-outs of the main burner and pilot burner have been experienced. Also, it appears that an interaction of the combustion process at the burner tip was caused by pressure pulsations, that is, system acoustical phenomena. The flame under certain conditions can be caused to pulsate in response to force pressure pulsations and can actually couple with the acoustical phenomena of the tube system to form a dynamically unstable or self-resonant system. Also, when the air controller was substantially closed, the oxygen in the system was quickly depleted causing the flame to more readily go unstable and/or to couple with the system acoustical phenomena.
The present invention is directed to providing a primary supply of air which is always available, but to use a variable air controller for controlling the overall excess oxygen by controlling only a secondary air flow. The present invention has the following advantages:
(1) It allows regulation of the air fuel ratio over the firing range for 3% to 5% of excess air,
(2) It reduces irregular burning and flame-out of the pilot when the main burner is not firing,
(3) It allows the use of a smaller variable air controller since only the secondary air is controlled and this increases the accuracy of the control,
(4) It reduces flame-out of the pilot and main burner when the burner is turned on,
(5) It provides the means of helping to dynamically decouple the combustion process and the fire tube/stack acoustics, and
(6) It eliminates sweating in the burner housing.